I use Options\Indexes\Exclude to keep some virtual disk from indexing. by adding the virtual disk to the Exclude folder, it show a folder of disk letter like z:\ K:\. but after some time, I find this Exclude List lost, I must add again.

I wonder if you store the setting of Exclude List in a temp folder. Because I always clean my PC using CCleaner (www.ccleaner.com). I don't know if there is any connection with this software.

Thank you, NotNul.
1、Do you have Store settings and data in %APPDATA%\Everything enabled? (under Menu:Tools > Options > General)
YES.
2、Do you have Run as administrator enabled? (also under Menu:Tools > Options > General)
Previous No. enabled it now.
3、How often does this happen? Every couple of minutes? Days?
Maybe Some months. I usually check it every day after I add a new path, no problem. But after some days or month, I will remember to check, sometime this happened. So, I am not sure there is a regular interval.
4、Do you think it might be related to restarting your computer and/or logging out/in?
No.
5、Do these virtual disk(s) always have the same drive letter(s)?
NO. the driver letter is not fixed. I use number as drive name, if the drive name is 201901, it show 201901(E:). Driver letter will change, next time it maybe change to 201901(F:). Driver name not changed, but Driver letter may change.
6、Do you use a roaming profile (profile is stored on a server)? If you don't know what this means: that's OK; then the answer is probably No.
No.
7、I am also interested in the (virtual) filesystem of these disks (NTFS, FAT, ...) but that comes in the "next round"
The filesystem of virtual disk is NTFS.

First of all: please revert back to the non-"run as administrator" settings. This is the best option for most people (my estimation: for 99% of the people):
- enable Everything Service adn disable Run as administrator (Menu:Tools > Options > General)

This will likely solve your issue:

Remove all the exclusions you made for your virtual disks (K:\, Z:\, ..)

Go to Menu:Tools > Options > Indexes > NTFS

Disable Automatically include new fixed volumes

Select your virtual disk in the Local NTFS volumes listbox

Disable Include in database

Repeat for all your virtual disks

That way new disks, including your virtual disks, will not be added automatically and you don't have to exclude them

When you do have a new NTFS disk that you want to add to the Everything database:

Go to Menu:Tools > Options > Indexes > NTFS

Select that disk in the Local NTFS volumes listbox

Enable Include in database

This part is optional:
I had suspicions that "something" deleted your entire Everything.ini from time to time, but based on your info that might not be the case. To keep an eye out, change the layout of the Everything main window a little so that if settings get deleted, that will be visible right away. A few candidates are

Show the Filters toolbar (Menu:View > Filters)

Change the color theme of Everything manually ( Menu:Tools > Options > Fonts and color ) or through this

Change the size of the search bar.
(For this, you have to exit Everything first:right-click the Everything icon in the system tray and choose Exit)
Edit your c:\users\roygao\Appdata\roaming\Everything\Everything.ini: change